While i can't inform you about SQL injections I can help you prevent php injections which can be much more harmfull to security ie:
That code can will make a url like yourdomain.com/index.php?page=yourpage then it will try to include yourpage.php if it doesn't exist you just get a 404 but using a php interjection you can modify the url to show your passwords from your server. By changing ?page=yourpage to ?page=.../etc/passwd it would normally look for .../etc/passwd.php and it would not find it causing a 404 but if you follow that with a php NULL charecter the signifies the end of the string {i will not write the NULL charecter for security reasons} it will display your password folder on what would normally be inaacessable via HTTP. You can fix this by using
Hope that helped...